In past decades, advances in synthetic strategies have resulted in the preparation of a variety of high-quality colloidal semiconductor nanocrystals with well-controlled size, shape, and surface passivation.1-3 These nanocrystals range from II-VI (e.g., CdSe and CdTe), IV-VI (e.g., PbS and PbSe), and III-V (e.g., InAs and GaP) semiconductors.4-10 Their novel properties have led to nanocrystals being used as biological fluorescent labels, chemical catalysts, separation reagents, structural building blocks, critical components in single-electron tunneling devices, solar cells, lasers, light-emitting diodes, as well as in many other applications.11-19 
The potential for using nanocrystals in a wide variety of applications has stimulated research efforts to develop synthetic methods to incorporate dopants into a variety of colloidal semiconductor nanocrystals.24, 29, 45, 47, 50, 58, 59, 60, 61, 62 It has been found that nanocrystals with dopants inside their crystal lattice can exhibit different properties from those with dopants on their surface.24, 29, 45, 47, 50, 58, 59, 60, 61, 62 
In bulk semiconductors, the ability to precisely control impurity doping has enabled most modern semiconductor applications.20 Doping with conventional impurities (donors and acceptors) allows the control of the number of carriers (electrons and holes) in semiconductors, which builds the foundation for p-n-junction-based semiconductor devices.21 In addition, doping with magnetic impurities (e.g., Mn) allows the production of paramagnetic or even ferromagnetic semiconductor crystals,22 which are important to spintronics applications.23 Compared with conventional charge-based devices, spintronic devices allow faster data processing, less power consumption, and higher integration densities.23 However, impurity doping in colloidal semiconductor nanocrystals remains to be fully mastered.24 
Despite decades of experience in doping bulk semiconductors with conventional impurities, the extension of such doping to semiconductor nanocrystals has proved very difficult. So far, n- and p-type doping of semiconductor nanocrystals by conventional methods has been unsuccessful in colloidal nanocrystals, in part, because of the difficulties in introducing the impurities.24 Alternatively, n- and p-type nanocrystals have been made by carrier injection.26,27 Such doped nanocrystals exhibit very high collective conductivity (e.g. ˜10−2 siemens per centimeter) in thin films.28 
Because of the difficulties in conventional-impurity doping, most efforts to date have focused on doping semiconductor nanocrystals with magnetic impurities.24 Such efforts are inspired by the progress in bulk diluted magnetic semiconductors (DMS), which are potentially useful in magnetic switching and spintronics.22,23 Typically, these magnetic impurities do not influence nanocrystal properties by introducing extra carriers, but by interacting with the quantum-confined electron-hole pair.24 In addition, these magnetic impurities can act as paramagnetic centers in the semiconductor lattice.
So far, a variety of II-VI and III-V semiconductor nanocrystals have been doped with magnetic impurities, such as Mn, Co, Ni, Eu, and Tb.29-48 A very large Zeeman effect, where atomic energy levels are split into a larger number of energy levels and the spectral lines are split into several components, has been observed in Mn-doped CdS and ZnSe nanocrystals.29,30 The interpretation of such a large Zeeman effect is that the quantum-confined electron-hole pair feels an effective magnetic field up to 400 Tesla, which is caused by the presence of a few Mn2+ ions in nanocrystals.24,29 
In addition to magnetic properties, some magnetic dopants (e.g., Mn2+ and Eu2+) can also introduce new luminescence properties to nanocrystals.29,41-43 Doping wide-gap II-VI semiconductor nanocrystals (e.g., ZnS and ZnSe) with these dopants can lead to the synthesis of nanocrystals with photoluminescence (PL) in the visible spectral region. These doped particles are much less toxic than the widely studied CdSe-based nanocrystals, and therefore they can be more important in nanocrystal-based applications such as biomedical diagnosis.65 However, the typical PL quantum yield (QY) of these doped nanocrystals is lower than that of CdSe-based nanocrystals.29 The low PL QY could limit the applications of these doped nanocrystals.12 To date, synthesizing doped nanocrystals with a high PL QY remains a challenge.
Two types of synthetic methods have been used to make doped nanocrystals.24 The first method is based on aqueous-phase coprecipitation or inverse micelle. This method often suffers from low crystallinity and broad size distributions.24 The second method is organic-phase high-temperature growth, which can produce monodisperse and highly crystalline colloidal nanocrystals.24 In many cases, the impurity atoms only exist at the surface of the nanocrystals but not inside the core, therefore minimizing the impurity's effects on the nanocrystal's properties.24 An isocrystalline shell-growth method has been introduced to incorporate these surface impurities inside the cores.46,47 Despite such progress, the synthesis of doped nanocrystals has not been fully understood. For example, manganese cannot be easily incorporated into a wurtzite CdSe nanocrystal even though manganese atoms have near 50% solubility in bulk CdSe crystals.48,49 Recently, Erwin et al. suggested that surface kinetics play a key role in impurity doping of nanocrystals.50 According to Erwin et al., the doping efficiency is determined by the initial adsorption of impurities on the nanocrystal surface during growth, and the binding energy of the impurity atom to specific surface facets is important to the adsorption.50 However, very recently Chelikowsky et al. has suggested that self-purification is an intrinsic property of defects in semiconductor nanocrystals, and nanocrystal shape is not critical for incorporating dopants.66 
Furthermore, impurity atoms are Just randomly absorbed into the nanocrystals during their growth.37,50 Therefore, impurity atoms are randomly located inside a nanocrystal made by the current syntheses.37,50 In addition, the current doping syntheses use a one-pot method.50 The impurity precursor (the precursor containing impurity atoms) and intrinsic precursor (the precursor for making the major semiconductor lattice of the nanocrystals) are mixed during the entire doping synthesis.24, 29, 30, 50 Therefore, impurity doping may occur during both nanocrystal nucleation and growth stages. The complexity of the nanocrystal nucleation4,6 makes it very difficult to control doping levels in such a synthesis. Specifically, a high concentration of the impurity precursor (e.g., for making nanocrystals with a high doping level) could lead to nucleation of pure dopant materials. The nucleation of pure dopant materials would result in a broad distribution of doping levels among the nanocrystals (the amount of impurity atoms varies between different nanocrystals) in the same growth solution.